Alkalide phosphors which are sensitive to ultraviolet radiation have been chiefly used to detect ultraviolet radiation. Also, photomultipliers are known as UV detectors. Although sensitive to UV light, they are unable to record information contained in UV light.
Also, a UV detector making use of a diamond has been proposed. This utilizes a change in the sensitivity of the diamond to UV light, i.e., a change in the electrical conductivity of the diamond caused by UV illumination. Thus UV detector is not capable of recording information contained in the impinging UV light.
On the other hand, a method exploiting photoluminescence has come to be known as a result of researches conducted recently. In this method, BaFBr doped with Eu or RbBr doped with T1 is illuminated with UV light or visible light. The amount of the illuminating light is read by illuminating BaFBr or RbBr with stimulating light consisting of infrared light. However, this method using photoluminescence is still in the research and development stage and has poor practicability.
The above-described photoluminescence can be roughly understood from a model described below. BaFBr or the like is doped with Eu that is a phosphor. UV light having energies greater than the bandgap energy of the phosphor is emitted, so that electron-hole pairs are generated inside the phosphor. Some of the electron-hole pairs are trapped in lattice defects and in the centers of capture of positive holes or electrons which are created by impurities. Since these centers of capture are located deep in the bandgap, electrons and positive holes captured in the centers are in stable state even at room temperature. The levels of the energies at which the centers of capture exist correspond to the energies of the light in the infrared region. Accordingly, light at these levels is directed as stimulating light to the phosphor. In this way, electrons and positive holes captured in the centers of capture recombine, thus emitting light.
However, when one attempts to measure the amount of the illuminating UV light by making use of the above-described phenomenon, the intensity of light emitted from the phosphor is so weak that the method is not practical. The intensity of the emitted light may be effectively increased by increasing the thickness of the phosphor film. If the phosphor film thickness is increased, the intensity of the illuminating UV light and the intensity of the stimulating light for reading the amount of illuminating UV light must be increased. If the intensity of the reading light is increased, local heating occurs. The produced heat diffuses to the surroundings. This deteriorates the resolution at which information is read out. If the thickness of the film of the phosphor is increased, scattering of the light is increased. This also leads to a decrease in the resolution at which information is read and written.